A surge of polar air is set to plunge temperatures across the southern United States, bringing unseasonably cold conditions to a vast portion of the country. This week, regions as far south as Arkansas, Tennessee, and Mississippi will experience below-normal temperatures, a stark contrast to the late summer warmth typically expected. Approximately 250 million Americans are bracing for mid-October-like conditions, with daytime highs ranging from the 40s to 80s Fahrenheit, depending on the specific locale.